1. Field of the Invention
The present invention is directed to a high power wireless telephone, and more particularly, to a high power digital cordless telephone with over-voltage protection that prevents receiver saturation.
2. Discussion of the Prior Art
Wireless telephones and base stations each has a radio frequency (RF) transmitter section and a receiver section, the combination of which is often referred to as a transceiver. The RF transmitter and receiver have been integrated on monolithic integrated chips (ICs) which are widely used in wireless communications. The receive section of a mobile telephone system is typically formed on a single IC and contains an input low noise amplifier (LNA), a mixer, and an oscillator. Such ICs containing transmitter or receiver sections provide low cost, low power consumption, and small size solutions for analog and digital cordless/wireless telephones. In addition, these ICs work very well at low input power signal levels.
One of the main methods to increase phone range is to increase its output power. However, power levels larger than a certain level overload the front end (input) RF circuits of the receiver. This occurs when the handset radio is too close to the base radio, for example, when the cordless handset is in the charging cradle. Overloading of the receiver (RF IC) causes various problems, such as increasing noise, degrading or even preventing a link between the handset and the base from being established, and increasing the bit error rate (BER) of the received signal.
The transmitter sections of both the handset and the base station have power control to increase transmission power when the distance therebetween increases, and to decrease transmission power when the wireless telephone is close to its base. One reason for power control is to prevent saturation or overload of the LNA located in the receiver of the handset or the base.
Power control is particularly important for high power cordless telephones having at least two handsets and one base, where each handset is at a different distance from the base. For example, one handset is on the base cradle for battery charging while the other handset is far away, such as a mile away for high power cordless phones. In this case, the base must transmit with high power to establish a link with the far handset. Without power control, the near handset receiver would be overloaded, thus increasing BER and even preventing establishment of a link between the near handset and the base.
In the case of an overloaded receiver, there are several options to establish a link. One option is increase the minimum distance between handset radio and base radio. Another option is to decrease the output power of the transmitter, and hence reduce the maximum range of the telephone. However, the reduced power may not be sufficient to establish a link with the far handset. A further option is to reduce the received RF input power level of the receiver by implementing special RF limiting circuits.
Different types of RF limiting circuits have been used in the input RF circuit of the receiver section, typically between the LNA and the antenna, to protect the input RF circuits by reducing the amplitude of received RF signal. Such RF limiting circuits include Shottky diode limiters, PIN diode limiters and ferrite limiters. However, these conventional RF limiting circuits add to the loss of the input RF circuits, thus reducing the levels of the received signals even when no reduction is desired.
The additional signal level loss resulting from conventional RF limiters decreases signal-to-noise ratio (SNR) of the receiver, thus degrading sensitivity and range of the telephone. Further, the conventional RF limiting circuits are large and consume valuable real estate on the board, thus preventing miniaturization of handsets and base stations. In addition, the conventional RF limiting circuits are costly and require relatively large power, thus increasing the telephone price and decreasing operation time before battery recharge is required. Accordingly, there is a need for an efficient power control which does not increase size and cost of wireless/cordless telephones and allows establishment of simultaneous links between a base and near and far handsets, with minimal degradation of SNR and BER.